This invention relates to the art of heating sheet material and, more particularly, to improved methods and apparatus for heating plastic sheet material.
In connection with the heating of plastic sheet material for processing, such as by forming the heated sheet in a press to form an article of manufacture, it is of course necessary to heat the plastic sheet over a predetermined soak time to elevate the sheet to the appropriate temperature for forming. In connection with a production line operation for forming articles of manufacture, such heating for a predetermined soak time has been achieved by progressively feeding sheet material along an elongate horizontal path through an oven and at a rate of travel providing for the sheet material to have the desired temperature and other forming characteristics upon reaching the exit end of the oven. The rate of the output of such an oven with respect to time is dependent on the length of the oven path whereby, in order to increase the production rate with respect to the article, an undesirably long oven path must be provided to facilitate a higher feed rate of the sheet material therethrough. Accordingly, an undesirable amount of floor space is required to accommodate the heating apparatus in order to obtain a suitable production rate. Further, while the feed rate of plastic material can be adjusted to provide the appropriate soak time for a given plastic material and/or the dimensions of the plastic sheets, there is no capability of selectively removing a sheet from within the oven prior to the soak time provided by feeding the sheet completely through the oven, or to increase the soak time for a given sheet other than by reintroducing the heated sheet following discharge from the oven thus doubling the soak time. Therefore, it is desired for example to experiment with the forming or other processing of heated plastic sheets of different plastic materials and/or sheet dimensions, it is necessary to heat such different sheets individually by movement thereof through the oven at a feed rate providing the desired experimental soak time, whereby such experimental use of the apparatus is both time consuming and expensive. While it might be possible to stop the feeding of sheets through such an oven to facilitate the removal of a given sheet prior to its reaching the exit end of the oven, this cannot be done without adversely affecting the heating of the other sheets within the oven.